Publications: Thesis

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Designing an enzyme able to catalyse a particular chemical reaction is a huge challenge. Different strategies adopted for this task have been developed in the last years. De novo design is one of these, where a
protein without specific catalytic properties is used as a scaffold for an active site design from scratch.
The work contained in this dissertation is focused on the theoretical investigation of the multi-step Retro-Aldol reaction mechanism of 4-hydroxy-4-(6-methoxy-2- naphthyl)-2-butanone (methodol) catalysed by three different previously published protein scaffolds; two de novo enzymes, RA95.5-8F and RA95.5-5, and one catalytic antibody, 33F12. Using different computational methods based on the use of multiscale QM/MM potentials, the first part of the study has aimed to understand in detail their reaction mechanism in terms of geometries and free energy landscape. These studies, and their comparison, allow us to identify the roles played by the amino acids around the different reaction sites, their interaction with the species involved in the reaction, and how they favour or not the stabilization of the transition states of the different chemical steps. The information derived from these studies has been exploited in the identification of specific amino acids that could be mutated in one of these protein scaffolds, the de novo enzyme RA95.5-8F, in order to improve its catalytic activity. Due to the multistep character of the reaction to be catalysed, this is a challenging goal since the activation free energy of the rate-determining step (RDS) of the reaction must be reduced but without harming the other chemical steps. As shown in the last part of this thesis, our investigation has successfully led to the proposal of a new catalytic protein for the catalysis of the Retro-Aldol reaction, with higher catalytic efficiency compared to the most efficient RA95.5-8F. This new strategy can significantly support and accelerate the experimental works on the design of new enzymes.

The development and use of clean, sustainable and safe energy sources, in order to substitute the use of fossil fuels, is a current challenge of science and technology. Solar energy, the only viable alternative, can be converted and stored in the form of molecular bonds, mimicking the photosynthesis process in green plants, to obtain fuels or other added-value products. This process requires semiconductor materials that can efficiently harvest and transform solar into chemical energy. In the present doctoral thesis, the study of semiconductor materials for photo-electrocatalytic applications was addressed from different approaches. That includes: the modification of photoelectrodes with catalytic coatings, obtained from a metal-organic framework; the implementation of a new method for the understanding of the photoelectrodes operating mechanisms; the integration of electrocatalytic and photovoltaic devices from Earth-abundant materials; and, finally, the investigation of new systems with potential application in photo-electrocatalytic processes.

Halide perovskite (HP) materials have recently attracted worldwide attention due to its fast progress in photovoltaic community. Photoconversion efficiencies (PCEs) of solar cells based on HP have increased quickly from 3.8% in 20091 to over 25%2 in 2019, for single junction architecture exceeding the maximum efficiencies achieved with CdTe (22.1%) and CIGS (22.9%).2 Moreover, HPs have been also applied not only for the preparation of high efficiency solar cells, but also photodetectors,3 light emitting diodes (LEDs),4-7 light amplifier8-9 and lasers.10-13 Such huge promising range of optoelectronic application is due to the outstanding versatility of HP materials. Structurally all components of HPs, with general chemical formula AMX3, can be easily changed/modified for adapting the specific requirement of a concrete optoelectronic application. For an example different dimensionalities of HPs can be obtained by controlling the size of an organic cation A. In addition to this versatility the success of HPs is based in the low non-radiative recombination, even for polycrystalline samples, due to a benign defect physics.14 Furthermore HPs can be prepared from solution methods at low temperatures and, consequently, using low cost fabrication techniques. Solution processes facilitate HPs to combine easily with other materials.

Combinations of materials with different nature has been a successful strategy in order to develop new materials with enhanced properties. As demonstrated in the past, adobe, stained glass and stainless steel are just some testimonies of great combined materials which have been supported for human life since centuries. This strategy is still useful nowadays when the development of chemistry, quantum mechanics and nanotechnology has created a revolution of material science. In the same strategy, the purpose of this thesis is to investigate the interaction of HP with other materials, in order to obtain properties and/or devices with enhanced functionalities and performance by the synergistic combination of different materials. Within the scope of this thesis, we have studied the interaction of methylammonium lead iodide (MAPI) HP with colloidal quantum dots (QDs), organic molecules and electron transport materials (ETMs).

QDs have been chosen for the combination with HP because QDs offer a huge versatility of optoelectronic properties such as tunable band gap, strong emission with highly pure color which are simply tuned by size or shape control due to the confinement effect. And QDs have been potentially applied in different optoelectronic devices such as LEDs15-16 and solar cells.17-18 Among the different semiconductor colloidal QDs, in this thesis we selected PbS QDs to study their interaction with HP because those materials possess a similar crystal structure with a relatively low lattice mismatch.19 The interaction of HP and QDs has been studied in the situation in which QDs were embedded in MAPI perovskite matrix. The presence of both PbS QDs and their capping ligands have a strong impact on the formation of HP. Small density of QDs intermixed in the precursor solution serves as nucleation centers promoting the growth of HP along a preferred direction. As a result, the morphological, optical and structural properties of HP were significantly improved. Consequently the performance of solar cells based on HP with embedded QDs was enhanced. Interestingly the properties and hybrid HP-QD films and those based devices were also dependent on the QD capping ligands. Moreover, the interaction between MAPI HP and QDs also resulted in a new property which is an emission of the exciplex state at energy lower than that of both HP and QDs.

Within the scope of this thesis we also studied the interaction of HP with organic molecules because organic molecules are very flexible materials. They present an unlimited number of structures that can be potentially synthesized and easily modified to meet the requirements of specific applications. In our study, organic molecules were introduced to HP films through the anti-deposition step. We have found that those organic molecules were preferentially located at the grain boundaries of HP. At the grain boundaries the structural disorders potentially can form defect states that can contribute to the degradation of the optoelectronic quality of the HP films. We show that the presence of organic molecules passivated efficiently those grain boundaries. Consequently, we obtained an improvement in performance of three different optoelectronic devices (solar cells, LEDs, and light amplifiers) based on HP with organic additives, comparing with references (without organic molecules).

Electron transport materials (ETMs) are very important in the performance of optoelectronic devices as they decide how efficient electrons can be extracted or injected. Additionally, in n-i-p configuration ETMs also have a crucial role on the formation of HP as it is directly deposited on ETM. It has been previously demonstrated that many factors of ETMs including nature,20 roughness,21 temperature,22 crystallinity23 and structure24 affecting on the formation and thermal stability of HP films. In this thesis, we studied the interaction of HP with spray-pyrolyzed ZnO ETMs. And we found that different termination of ZnO surfaces, obtained by different gas component used during the spray pyrolysis, not only influences the formation of fresh MAPI HP films but also their evolution during storage and finally impacts in the long-term stability of device performance. As a result, our HP solar cells based on ZnO ETMs showed not only a good stability but also an improvement in performance even after more than one month of preparation, under the storage condition of 35% humidity.

Through the study of the interaction of HP with three different materials and in three different situations where the interaction takes place including underneath, inside and up surface of HP layers, we demonstrate that combining HP with other materials of different nature is a promising strategy in order to create new material composites with enhanced properties which strongly influence, in turn, the optoelectronic development.
References

1. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. Journal of the American Chemical Society 2009, 131 (17), 6050-6051.
2. NREL photovoltaic efficiency chart: https://www.nrel.gov/pv/cell-efficiency.html
3. Dou, L.; Yang, Y.; You, J.; Hong, Z.; Chang, W.-H.; Li, G. Solution-processed hybrid perovskite photodetectors with high detectivity. Nat Commun 2014, 5.
4. Jaramillo-Quintero, O. A.; Sanchez, R. S.; Rincon, M.; Mora-Sero, I. Bright Visible-Infrared Light Emitting Diodes Based on Hybrid Halide Perovskite with Spiro-OMeTAD as a Hole-Injecting Layer. The journal of physical chemistry letters 2015, 6 (10), 1883-1890.
5. Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright light-emitting diodes based on organometal halide perovskite. Nat Nano 2014, 9 (9), 687-692.
6. Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science 1996, 271, 933-937.
7. Lin, K.; Xing, J.; Quan, L. N.; de Arquer, F. P. G.; Gong, X.; Lu, J.; Xie, L.; Zhao, W.; Zhang, D.; Yan, C.; Li, W.; Liu, X.; Lu, Y.; Kirman, J.; Sargent, E. H.; Xiong, Q.; Wei, Z. Perovskite light-emitting diodes with external quantum efficiency exceeding 20 per cent. Nature 2018, 562 (7726), 245-248.
8. Suárez, I.; Juárez-Pérez, E. J.; Bisquert, J.; Mora-Seró, I.; Martínez-Pastor, J. P. Polymer/Perovskite Amplifying Waveguides for Active Hybrid Silicon Photonics. Advanced materials 2015, 27 (40), 6157-6162.
9. Suárez, I.; Hassanabadi, E.; Maulu, A.; Carlino, N.; Maestri, C. A.; Latifi, M.; Bettotti, P.; Mora-Seró, I.; Martínez-Pastor, J. P. Integrated Optical Amplifier–Photodetector on a Wearable Nanocellulose Substrate. Advanced Optical Materials 2018, 6 (12), 1800201.
10. Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D.-D.; Higler, R.; Hüttner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; Atatüre, M.; Phillips, R. T.; Friend, R. H. High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. The journal of physical chemistry letters 2014, 5 (8), 1421-1426.
11. Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.; Jin, S.; Zhu, X. Y. Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors. Nature materials 2015, 14 (6), 636-642.
12. Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Low-temperature solution-processed wavelength-tunable perovskites for lasing. Nature materials 2014, 13 (5), 476-480.
13. Stranks, S. D.; Hoye, R. L. Z.; Di, D.; Friend, R. H.; Deschler, F. The Physics of Light Emission in Halide Perovskite Devices. Advanced materials 2018, 0 (0), 1803336.
14. Wang, F.; Bai, S.; Tress, W.; Hagfeldt, A.; Gao, F. Defects engineering for high-performance perovskite solar cells. npj Flexible Electronics 2018, 2 (1), 22.
15. Shirasaki, Y.; Supran, G. J.; Bawendi, M. G.; Bulovic, V. Emergence of colloidal quantum-dot light-emitting technologies. Nat Photon 2013, 7 (1), 13-23.
16. Pal, B. N.; Ghosh, Y.; Brovelli, S.; Laocharoensuk, R.; Klimov, V. I.; Hollingsworth, J. A.; Htoon, H. 'Giant' CdSe/CdS core/shell nanocrystal quantum dots as efficient electroluminescent materials: strong influence of shell thickness on light-emitting diode performance. Nano letters 2012, 12 (1), 331-6.
17. Nozik, A. J. Quantum Dot Solar Cells. Physica E 2002, 14, 115-200.
18. Lan, X.; Voznyy, O.; García de Arquer, F. P.; Liu, M.; Xu, J.; Proppe, A. H.; Walters, G.; Fan, F.; Tan, H.; Liu, M.; Yang, Z.; Hoogland, S.; Sargent, E. H. 10.6% Certified Colloidal Quantum Dot Solar Cells via Solvent-Polarity-Engineered Halide Passivation. Nano letters 2016, 16 (7), 4630-4634.
19. Ning, Z.; Gong, X.; Comin, R.; Walters, G.; Fan, F.; Voznyy, O.; Yassitepe, E.; Buin, A.; Hoogland, S.; Sargent, E. H. Quantum-dot-in-perovskite solids. Nature 2015, 523 (7560), 324-328.
20. Olthof, S.; Meerholz, K. Substrate-dependent electronic structure and film formation of MAPbI3 perovskites. Scientific reports 2017, 7, 40267.
21. Climent-Pascual, E.; Hames, B. C.; Moreno-Ramírez, J. S.; Álvarez, A. L.; Juarez-Perez, E. J.; Mas-Marza, E.; Mora-Seró, I.; de Andrés, A.; Coya, C. Influence of the substrate on the bulk properties of hybrid lead halide perovskite films. Journal of Materials Chemistry A 2016, 4 (46), 18153-18163.
22. Zhang, H.; Zhao, C.; Li, D.; Guo, H.; Liao, F.; Cao, W.; Niu, X.; Zhao, Y. Effects of substrate temperature on the crystallization process and properties of mixed-ion perovskite layers. Journal of Materials Chemistry A 2019, 7 (6), 2804-2811.
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24. Grancini, G.; Marras, S.; Prato, M.; Giannini, C.; Quarti, C.; De Angelis, F.; De Bastiani, M.; Eperon, G. E.; Snaith, H. J.; Manna, L.; Petrozza, A. The Impact of the Crystallization Processes on the Structural and Optical Properties of Hybrid Perovskite Films for Photovoltaics. The journal of physical chemistry letters 2014, 5 (21), 3836-3842.

Perovskite solar cells (PSC) have emerged as an important player in the search for producing c1ean and renewable energy at high efficiencies. However, there exist several bottlenecks to maximise their efficiency and stability. This thesis explores the physical mechanisms related to these limitations in operation and provides a deeper understanding as to how to overcome them. Through systematic experiments based on the small perturbation methods of Impedance Spectroscopy (IS) and Intensity Modulated Photocurrent Spectroscopy (IMPS), the perovskite/selective contact interfaces are identified as a critical factor that controls charge accumulation, recombination ando extraction. Therefore, these interfaces require to be carefully tuned in order to gain control over the device operation. Based on these insights, a robust equivalent circuit of the PSC is developed that provides a strong foundation for further development of these solar cells

The present thesis is divided in two different parts. Part 1 is titled the importance of ligand design for the development of supramolecular catalysts. This part includes three different chapters: introduction chapter 1, chapter 2 and chapter 3. Chapter 1 shows a brief overview of the most interesting items related to supramolecular catalysis. In chapter 2 is described the synthesis and characterization of three different p-xylylbis-benzimidazolylidene iridium and rhodium complexes. Chapter 3 reports the synthesis, characterization and catalytic studies of different palladium, iridium and rhodium complexes, which are formed by N-heterocyclic ligands featuring different topologies, and some of them decorated with pyrene functionalities. The importance and influence of these ligands in the conformational and catalytic behaviour of the metal complexes is studied in detail, providing evidences of the effects produces due to non-covalent interactions such as π-π interactions. Part 2 is titled the importance of ligand design for the development of ion receptors. This part includes three chapters: introduction chapter 4, chapter 5 and chapter 6. Chapter 4 is a brief introduction of the most relevant aspects related to supramolecular host-guest chemistry. The approaches described in chapter 5 consist of two different strategies for the preparation of imidazole resorcinarene based cavitands for the recognition of anions or cations. Chapter 6 reports the synthesis of tris-azolium and tris-iodoazolium tripodal receptors for the recognition of anions. In both chapters (5 and 6) are studied the binding capabilities of the receptors towards several ions, showing the importance of the development in ligand design to improve the properties of the receptors.